Stephen Stearns, teaching Yale's Evolution and Medicine course, uses this session to explain how randomness shapes evolutionary outcomes. Drawing on the textbook Evolutionary Medicine, he distinguishes chance events like genetic drift and mutation from the deterministic pressure of natural selection, showing how both operate together in populations. Stearns works through the logic of why evolution is not a strictly optimizing process, using population genetics concepts to illustrate how random sampling of alleles across generations can push traits in directions selection alone would not predict. The lecture sets up a conceptual foundation for the rest of the course, which applies evolutionary reasoning to human health and disease. At twenty one minutes, it stays focused on a single idea, delivered in Stearns's plain, classroom style with minimal visual aids beyond basic diagrams.